Automatic tuning electric system



1949- w. P. BOOTHROYD AUTOMATIC TUNING ELECTRIC SYSTEM Filed Oct. 4, 1944 d R w m I m f R V .M m m 5 v M 0 w ,M. B M Y B SQQIQI W Patented Nov. 1, 1949 UNITED PATENT cert-cs 2;.486551 AUTIIOMA'HC .TIINING ELECTRIC sxsrEM Wilson P. '-Boothroyd,i]?hiladelphia, 1a., assignor,

lby zm'esne assignments, to lliilco @orporaition, Philadelphia, sPa., a. corporation :of lliennsylmania.

.8 Claims.

vThe :present invention relates 1:0 an automatic frequency control circuit and particularly to such a circuit for sweeping a lban'd o'f frequencies and locking .the converter to the first carrier received within ztheswept .over band.

receiver .to which this circuit has been applied is designed for the reception of continuous :waue frequency modulated signals, .although qu'ite obviously the circuit could be utilized in other types .of receivers.

.Priorcircuit arrangementsrutilized for the :same general purposef'have been unsatisfactory in "that they :required extra aelectron :tubes :or llacke'd sensitivity or would not operate at .low :sweep rates without -:the :use of :large coupling components in the :circuit.

has been stated above the circuit is particularly useful in conjunction with in :narrow band superheterodyne receiver 'used to receive signals irom :a plurality -:-of transmitters within a-specified frequency band. In order :to :avoid problems of frequency stability and .the necessity of making absolute frequency measurements :on

several rmicrowave transmitters :and neceivers so that the transmitters would FHJWSZYS ftransmit'rsignals within "the narrow ban'd' of the :receivers. means are 1providedzfor the completely automatic control of iilocal oscillator frequency ro'f ithe receiver.

By :means of this arrangement rthcllocal :oscillator'frequencyiislcaused .tocsweep :over the transmission'band, to cut off the sweep upontthen'eceip't of 1a :carrierfrequency :and to lock-the local oscillator ito the proper frequency forrreception of "signals "on that :carrier.

It is an object :of this invention to rprovide :an automatic frequency control circuit :which includes means for sweeping the local oscillator Ifarequency over :a relatively narrow transmission and.

Itiiszanoth'er object .o'f' the invention to provide such a sweep circuit in which the sweeping action will be stopped upon "reception of a carrier frequency vwithin the assigned band.

It is a 'further object of the intention =.to Jack the local oscillator to the incoming signal whereby the local osc'illator frequency :is the proper one for heterodyning the RE. center frequency to the;I-.F..center frequency ofthe received -g-transmission of the particular received ssignal.

Other objects and features of the invention will be apparent when the "following description is considered in connectionwith the appended drawing in which the single figure of the :drawings is 'a schematic diagram of :a :sweeping and locking circuit in accordance with my invention.

Referring now to the drawing, there is shown at in the last intermediate frequency tube :of the intermediate frequency amplifier ".from which intermediate frequency signals are fed to tthe limiter tube 1| I. It is preferred that "the 'intermediate frequency amplifier have suflici'ent gain to "produce two volts of 'noise .on the limiter grid, that is to say, two :volts of negative zrgrid bias in the absence of a :carrierfsignal.

The intermediate frequency output of the limiter H is applied over the coupling :circuit I32 to the discriminator :IB iwhich discriminatorlis tuned to the intermediate frequency and :also for noise balance, i. e., rzero direct :current rdiscr'iminator output for no carrier .wave input. The audio output of the discriminator is applied to an audio amplifier as :is indicated in the drawing. The circuits 5S0 far described are 'com monly utilized and consequently need not Joe described in greater detail.

vA local-oscillatorftube =l-4eis gprovidetl, the signal therefrom being :applied .in a normal manner to the mixer tube in which-the sincomingicarrier frequency is .heterodyned with the local oscillator frequency "to produce the intermediate frequency signal Etor :application :to the first stage :of intermediate frequency amplification. Tube M is a velocity modulated .McNally tube such, for example,stype:i2K28. Such .a tube, 1 as is well known, can he frequency modulated by means of the tube .repeller electrode, that :is the tube can be modulated by variations inthe wvoltage applied to the repeller.

For normal automatic frequency rcontrolon a carrier the direct current output .of thediscriminator :13 is connected to a directcurent amplifier t5, the Eplate circuit of which controls the local oscillator repeller .and hence, the local oscillator frequency. Initially, the local oscillator repeller-circuit is adjusted, by adjusting the cathode resistor :l6 of the direct currentlan' plifier lli toggive proper plate voltage from the amplifier with zero signal .fromthediscriminator ll 3.

the plate resistor l8 of the cut off tube 20. The

cathode bias control 2| of the cut off tube 20 is adjusted so that, when the normal bias from the limiter, in the absence of carrier current, is present there is approximately 100 volts drop across its plate resistor 18. There is thus provided 100 volts for the plate circuit of the sweep generator H, the bias of which is so adjusted as to cause it to oscillate at this voltage. Obviously, the values given may be altered and the above is merely a typical example of values which have been satisfactory.

The condition mentioned above is the normal rest condition of the circuit and provides for the sweeping of the receiver through a desired reception band.

When a carrier is found by the sweeping oscillator the signal in the limiter l2 increases above the noise and consequently the bias derived from the grid of the limiter tube I increases. This bias, applied over lead 22 to the grid of cut-off tube 20 causes a decrease in the plate current of the cut-off tube 20 thereby lowering the voltage across the sweep generator I? and causing it to stop oscillating. Therefore, the sweep voltage is cut off whenever a carrier is present and the cal oscillator is then looked on the incoming signal by the automatic frequency control circuit above described.

In addition to functioning as a cut-off tube to control the sweep generator, the tube amplifies the change in limiter bias and thus provides a high degree of control over the sweep generator.

A switch 23 is provided in the cathode circuit of the cut-off tube 20 which switch is normally closed but may be opened in order to bias the cut off tube and thereby cause the sweep generator i! to stop Oscillating thereby removing the sweep from the local oscillator so that the local oscillator circuit may be adjusted.

Of course, audio modulation must be kept from the cut-off tube and from the amplifiers and this may be done by the utilization of suitable well known filters.

While I have described a preferred embodiment of my invention it is obvious that other embodiments and constructions may be adopted without departing from the spirit thereof and, consequently, I desire to be limited not by the foregoing description but solely by the scope of the appended claims.

What is claimed is:

1. In a superheterodyne receiver adapted to receive signals within a band of frequencies and having an oscillator for supplying heterodyne frequencies, means for automatically sweeping the tuning of the receiver over the frequency band comprising, in combination, a relaxation tube oscillator, means to bias said oscillator to sustain oscillations thereof when no signal is received, an electron tube, said tube supplying the bias to said relaxation oscillator, means to apply the output of said relaxation oscillator to the heterodyne oscillator and thereby to cause said heterodyne oscillator to sweep over the frequency band and thereby tune the receiver in a repeated sequence to each frequency within the band and means operable upon the reception of a signal within the band to control said electron tube and decrease its output thereby reducing the bias of said relaxation oscillator and causing it to cease to operate.

2. In a superheterodyne receiver adapted to receive signals within a band of radio frequencies,

Number Name Date 2,287,925 White June 30, 1942 2,410,075 Hutchins Oct. 29, 1946 FOREIGN PATENTS Number Country Date 549,770 Great Britain Dec. 7, 1942 549,771 Great-Britain Dec. 7, 1942 said receiver having a local oscillator and a frequency discriminator, the combination comprising means for varying the frequency of said local oscillator in response to a voltage, means for applying theoutput of said discriminator to said frequency varying means, a sweep voltage generator operable in response to an actuating voltage of at least a predetermined magnitude, control means for developing said actuating voltage, means for applying said actuating voltage to said generator, means for applying the output voltage of said generator to said frequency varying means to cause said local oscillator to sweep over a predetermined frequency band and thereby cyclically tune said receiver to frequencies within said radio frequency band, and means connected to said control means and responsive to a received signal for reducing the output voltage of said control means below said predetermined magnitude, whereby said sweep generator ceases operation and said frequency varying means, in response to the output voltage of said discriminator, controls the frequency of the local oscillator so as to properly receive and hold the received signal.

3. In a superheterodyne receiver adapted to re ceive signals within a band of radio frequencies, said receiver having a local-oscillator, a frequency discriminator and a limiter, the combination comprising means for varying the frequency of said oscillator in response to a voltage, means for applying the output voltage of said discriminator to said frequency varying means, a sweep voltage generator operable in response to an actuating voltage of at least a predetermined magnitude, an electron tube having an input and an output circuit, said output circuit supplying said actuating voltage to said generator, means for applying the output voltage of said generator to said frequency varying means, and means coupling said limiter to said input circuit, said means normally supplying a bias voltage to the input circuit in accordance with the noise level in said ilmiter to develop in the output circuit of said tube said actuating voltage of at least a predetermined magnitude to cause said local oscillator to sweep over a predetermined frequency band and thereby cyclically tune said receiver to frequencies within said radio frequency band, said coupling means upon reception of a signal within said radio frequency band supplying an increased bias voltage such that the output of said electron tube falls below said predetermined magnitude, whereby said generator ceases operation and said frequency varying means, in response to the output voltage of said discriminator, controls the frequency of the local oscillator so as to properly receive and hold the received signal.

WILSON P. BOOTHROYD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

